The proposed study has two overarching aims: (1) to advance theory on the spatial dynamics of neighborhood exposure for understanding youthful health risk behavior;and (2) to model adolescent participation in high risk marijuana use (early onset and high frequency use) and co-morbid sexual/HIV risk behavior employing state- of-the-art GIS-derived measures of urban space and time use patterns, and recently developed hierarchical Bayesian models for multilevel and spatial data. Central to our analysis of the impact of neighborhood exposure is theoretically defining and empirically exploring the concept of social isolation as evident in space- time activity patterns at both the individual and neighborhood levels, and exploring the implications of this concept for youthful drug use and co-morbid HIV risk behavior. Social isolation represents the extent and character of connections between a focal individual or neighborhood and other communities. Individuals are potentially linked to a wide range of communities through regular activities such work, school, child care, health care, informal socializing, and religious activities. These patterns of non-residential community exposure will vary across individuals and, in the aggregate, across communities in ways that may be important for understanding variation and disparities (by gender, race-ethnicity, and immigrant status) among adolescents in the onset and frequency of using marijuana, and the extent to which youth engage in risky sexual activity. To address these questions, we use data from the Los Angeles Family and Neighborhood Survey (L.A. FANS) along with census data for the neighborhoods in which youth and their families reside as well as travel to through the course of their day. L.A. FANS includes information for a diverse sample of youth, their families, and neighborhoods within 65 randomly selected census tracts in Los Angeles County. The analyses focus on all persons in the sample between the ages of 12 and 17. Our analytic strategy relies on geospatial measurement techniques, geovisualization, and hierarchical Bayesian models for multilevel and spatial data to explore key hypotheses. Findings from the study have the potential to broaden understanding of the role of context in organizing problematic youthful marijuana use and co-morbid sexual/HIV risk behavior, the consequences of which may ramify throughout, or truncate, the life course. The proposed research will contribute to a better understanding of how the different neighborhood environments that youth and their families move through during the course of their daily lives affect high risk marijuana use and risky sexual activity among adolescents. These activities are of concern for public health because they put youth at risk for physical and mental health problems in the current stage of their lives and into the future. Findings from the research may suggest the extent to which interventions to address adolescent health risks should target neighborhoods that are the most isolated from contact with other areas, and whether such interventions would be more influential for some youth than others (girls versus boys, youth of color versus whites, immigrants versus non-immigrants).